<?php
$priv=$_GET['priv'];
$steam=$_GET['stid'];
//Заполни сам, пожалуйста дальше
//ID привелегии => цена
$prices = [
	"vip" => 5,
    "prem" => 10,
    "creat" => 15
];
if(isset($prices[$priv]) and isset($steam)){
	echo "https://www.oplata.info/asp2/pay_wm.asp?id_d=2400801&lang=ru-RU&s=".$steam."&unit_cnt=".$prices[$priv];
} else {
	echo "ERR";
}





JS

function go() {
		var e = document.getElementById("group"),
			selected = e.value,
			res
		var req = new XMLHttpRequest()
		req.open('GET', "pay.php?priv="+selected+"&stid=76561198083372481", false)
		req.send()
		if (req.status != 200) {
		  	alert( req.status + ': ' + req.statusText )
		} else {
		  	res = req.responseText
		  	alert(res)
		}
		if(res != "ERR")
			window.open(res, '_blank');
		else
			alert("Ошибка!")
	}